Paget's disease of bone represents the most flagrant example of disordered bone remodeling in which abnormal osteoclasts demonstrate increased levels of bone resorption followed by an abundance of new bone formation. The primary cellular abnormality occurs in the osteoclasts. A possible viral etiology has been suggested for Paget's disease, because the osteoclasts express paromyxovirus nucleocapsid antigens, contain viral and cytoplasmic nuclear inclusions, and have been shown to express viral messenger RNAs by in situ hybridization studies. However, the mechanism responsible for the abnormal bone remodeling in Paget's disease have not been clearly defined. In this proposal we will use our recently described model for pagetic osteoclast formation to further characterize the mechanisms responsible for the abnormal osteoclast formation in Paget's disease. We have recently reported that multinucleated cells that have the characteristics of pagetic osteoclasts are formed in long term marrow cultures of marrow mononuclear cells from patients with Paget's disease. These characteristics include increased cell number and cell size, increased tartrate resistant acid phosphatase activity, increased sensitivity to 1,25 dihydroxyvitamin D3, increased rates of formation and ultrastructural features that are characteristic of pagetic osteoclasts. These multinucleated cells express measles and RSV nucleocapsid antigens similar to pagetic osteoclasts, and conditioned media produced by these cells contains factors that stimulate osteoclast-like cell formation in normal marrow cultures. In addition, we have found that immature cells in the osteoclast lineage are increased in patients with Paget's disease and have begun a preliminary characterization of these precursor cells. Thus, this model system should allow us the unique opportunity to examine in detail the pathophysiology of Paget's disease. In this proposal we plan to: 1) prepare a cDNA library from pagetic MNC in order to identify, sequence and clone viral cDNA's expressed by these cells and to characterize factors produced by pagetic MNC that enhance osteoclast formation; 2) determine the role of the marrow microenvironment in the increase of early osteoclast precursors, and the effects of the microenvironment on MNC formation; 3) identify and extensively characterize the precursor for the MNC in pagetic marrow and 4) establish an animal model of Paget's disease.